1. Field of the Invention
This invention relates to a method of recording and reproducing a radiation image by use of a stimulable phosphor sheet. This invention particularly relates to a radiation image recording and reproducing method wherein a radiation image stored on a stimulable phosphor sheet is read out without being adversely affected by a stimulated light emission after-glow.
2. DESCRIPTION OF THE PRIOR ART
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then scanned with stimulating rays which cause it to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved image quality, particularly a high diagnostic efficiency and accuracy.
FIG. 1 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system.
In the radiation image read-out apparatus of FIG. 1, a laser beam 1a of a predetermined intensity is emitted as stimulating rays from a laser beam source 1 to a galvanometer mirror 2. The laser beam la is deflected by the galvanometer mirror 2 to form a laser beam lb impinging upon a stimulable phosphor sheet 3 disposed below the galvanometer mirror 2 so that the sheet 3 is scanned by the laser beam 1b in the main scanning direction, i.e. in the width direction of the sheet 3 as indicated by the arrow A. While the laser beam lb impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 9. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1b.
As the stimulable phosphor sheet 3 is scanned by the laser beam 1b, the portion of the sheet 3 exposed to the laser beam 1b emits light having an intensity proportional to the stored radiation energy. The light emitted by the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its front end face 4a disposed close to the sheet 3 in parallel to the main scanning line. The light guide member 4 has a flat-shaped front end portion 4b disposed close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its front end face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5 via a filter (not shown) for selectively transmitting the light emitted by the sheet 3. Thus the light emitted by the stimulable phosphor sheet 3 in proportion to the radiation energy stored thereon is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or stored on a magnetic tape 8, or directly reproduced as a hard copy on a photographic film or the like.
In this manner, the radiation image stored on the stimulable phosphor sheet 3 is read out. However, since the front end face 4a of the light guide member 4 extends approximately over the overall width of the stimulable phosphor sheet 3 in parallel to the main scanning line thereon, all light emitted by the portions of the stimulable phosphor sheet 3 covered by the front end face 4a enters the light guide member 4 from the front end face 4a and is detected by the photomultiplier 5. That is, not only the light emitted by the portion of the stimulable phosphor sheet 3 upon which the laser beam 1b impinges at any given instant, in proportion to the radiation energy stored on that portion, but also the other light emitted as described below by the portions of the sheet 3 covered by the front end face 4a enters the light guide member 4 and is detected by the photomultiplier 5. The light other than the light emitted by the portion of the stimulable phosphor sheet 3 upon which the laser beam 1 impinges at any given instant in proportion to the radiation energy stored on that portion embraces after-glows emanated by the stimulable phosphor sheet 3. The after-glows are divided into an instantaneous light emission after-glow and a stimulated light emission after-glow.
By "stimulated light emission after-glow" is meant the after-glow of light emitted by a stimulable phosphor sheet carrying a radiation image stored thereon when the sheet is exposed to stimulating rays (for example, a laser beam) for reading out the radiation image, the after-glow continuing to be emanated by the sheet while the light intensity decays after the exposure of the sheet to the stimulating rays is ceased. The characteristics of the stimulated light emission after-glow are generally as shown in FIG. 2, though they will differ depending on the type of the stimulable phosphor constituting the stimulable phosphor sheet. In the graph of FIG. 2, the ordinate represents the intensity of light emission and the abscissa represents the time (t). As shown in FIG. 2, when the exposure of a stimulable phosphor sheet to stimulating rays is ceased after the sheet is exposed to the stimulating rays for a period of .DELTA.t from a time t1 to a time t2, the intensity of light emitted by the sheet upon stimulation thereof at a light emission intensity A does not immediately decrease to zero, but instead a stimulated light emission after-glow continues while the intensity thereof decreases along an exponential function curve with the time constant thereof increasing gradually. (That is, the light intensity decreases rapidly at the beginning and thereafter the rate of decrease in the light intensity becomes gradually lower.)
For example, decay of the light emission intensity of the stimulated light emission after-glow is such that the initial time constant is approximately one microsecond, i.e. the time t3-t2 required for the light emission intensity to become 1/e (B/A=1/e) is approximately one microsecond. In general, since the speed of scanning (in the main scanning direction) of a stimulable phosphor sheet by stimulating rays by use of a galvanometer mirror is approximately 50 Hz, it takes approximately 20,000 microseconds for one scanning. Accordingly, the intensity of the stimulated light emission after-glow decaying along an exponential function curve with the initial time constant of one microsecond becomes very low as compared with the intensity of the light emitted by the stimulable phosphor sheet upon stimulation thereof when the sheet is exposed to the stimulating rays. Thus the intensity of the stimulated light emission after-glow at each point of the stimulable phosphor sheet becomes almost negligible.
However, the light emission by the stimulable phosphor sheet upon stimulation thereof when the sheet is exposed to stimulating rays arises from a portion having a very small area upon which the stimulating rays impinge, whereas the stimulated light emission after-glow is emanated by the whole surface of the stimulable phosphor sheet scanned by the stimulating rays. Therefore, as the stimulable phosphor sheet 3 is scanned point by point by the laser beam 1b as shown in FIG. 1, the light emitted by a portion of the sheet 3 upon which the laser beam 1b impinges at any given instant in proportion to the radiation energy stored on that portion and the stimulated light emission after-glow emanated by all of the portions covered by the front end face 4a of the light guide member 4 simultaneously enters the light guide member 4 from the front end face 4a and is guided to the photomultiplier 5. In this case, since the area of the portions covered by the front end face 4a of the light guide member 4 is markedly larger than the area of the portion of the stimulable phosphor sheet 3 which is momentarily exposed to the laser beam 1b and which emits light upon stimulation by the laser beam 1b, the amount of the stimulated light emission after-glow guided to the photomultiplier 5 becomes not negligible even though the intensity of the stimulated light emission after-glow becomes negligibly low as compared with the intensity of the light emitted by the sheet 3 upon stimulation thereof. The stimulated light emission after-glow detected together with the light emitted by the stimulable phosphor sheet upon stimulation thereof by stimulating rays as mentioned above constitutes a noise component in the electric image signals obtained by the read-out of a radiation image and makes it difficult to accurately read out the radiation image. The stimulated light emission after-glow give rise to a problem concerning noise particularly when the scanning speed of stimulating rays on the stimulable phosphor sheet carrying the radiation image stored thereon in the course of image read-out is increased.